The present invention relates to electromagnetic interference (EMI) filtering, and in particular to a system and method for filtering EMI using active damping with frequency dependant impedance.
Three-phase inverters are widely used in industrial applications, particularly for driving electric motors. Inverters include at least one semiconductor device, such as a metal-oxide-semiconductor field-effect transistor (MOSFET) that operates in a switching mode at a given frequency. High switching speeds of the transistors lead to better power quality for the inverter, but also lead to greater electromagnetic interference (EMI).
For motor drive applications designated as “class A” industrial applications, the common-mode EMI is categorized into two types: conducted type and radiated type. Conducted type EMI consists of frequencies between approximately 150 kHz and 30 MHz. Radiated type EMI consists of frequencies beyond 30 MHz. Various EMI standards and specifications are in place to regulate EMI for these applications, particularly conducted type EMI.
In order to combat the effects of EMI from the power inverter, passive filters have been adopted between the inverter and the load, and/or between the direct current (DC) power supply and the inverter. Generally, passive filters employ an inductor-capacitor (LC) filter with a low-pass characteristic. These LC filters have a natural resonance frequency. Because of this, damping resistors have traditionally been employed to counteract the effect of this resonance.